Some Very Long Telephone Circuits of the 

 Bell System 



By H. H. NANCE 



RKCKXT papers' ha\e discussed at length the use of toll cables 

 for the handhiiK f>f certain long distance trartic. These cables, 

 which are i)eins used in areas of dense traffic, have been made possible 

 by many (le\elopments in cable design, repeaters and loading coils. 

 Coincident with these developments have gone others which are 

 finding their application in the extensive establishment of improved 

 open wire circuits for use over very long distances. The purpose of 

 the present paper is to discuss some of the considerations involved 

 in the overall design and maintenance of these very long open wire 

 circuits. These circuits are often referred to as "backbt)ne" circuits 

 and supply a network of trunk lines for the entire Bell Sy>tttii. The 

 nuist important of these routes are shown in Vig. I. 



The first transcontinental line was completed in the sunmier of 

 1914 and early in the following year three transcontinental telephone 

 circuits were placed in commercial service. These circuits were con- 

 structed of copper wire 105 mils in diameter loaded with 2')() millihenry 

 coils at intervals of 7.88 miles and had telephone repeaters located 

 at points about .500 miles apart. The opening of these first circuits, 

 while marking a most important stage in the progress of long distance 

 telephony, has been followed by many developments which ha\e 

 made possible increased overall transmission efficiency and improved 

 quality. A discussion of these developments is given in a paper on 

 "Telephone Transmission 0\er Long Distances," by H. S. Osborne. - 



Two outstanding characteristics of these new open wire circuits are 

 that they are non-loaded and that the repeaters are of an improved 

 type, the number being increased in consequence of the higher attenu- 

 ation. With these long non-loaded circuits increased speed of propa- 

 gation and smoother characteristics are obtained resulting in less echo 

 effect and better volume. Better attenuation-frequency character- 

 istics are obtained and the quality is further improved due to the 

 elimination, to a large extent, of transients. Changes in line attenua- 



' " I'hilatlclphia-Pittsburgh Section of New ^'ork-Chicago Cable," l)y J. J. I'illiixl, 

 Bfll .System Technkal Journal, Vol. I. No. 1. July. 1922; Journal of A.f.E.E., .XukusI, 

 Xtll. "Telephone Transmission Over Long ("able ("irniits," by .A. B. Clark, 

 Journal A./.E.E., January, 1923; Bell System Technical Journal, '\'o\. II, No. 1, 

 January, 192.V 



-For detail discussion sec paper on "Telephone Transmission Over Long Dis- 

 tances," by H. S. Osborne. Journal A.I.E.E., Vol. XI. 11, No. 10. Octolwr, 1923. 



495 



